RAGING hormones during pregnancy prompt mood swings, but may also lead to a heightened ability to recognise threatening or aggressive faces. This may have evolved because it makes future mothers hyper-vigilant, yet it could also make them more vulnerable to anxiety.

Previous studies have suggested that a woman's ability to correctly identify fearful or disgusted facial expressions varies according to her stage of the menstrual cycle, with perception heightened on days associated with high levels of the hormone progesterone. Since levels of progesterone and other hormones rise dramatically in late pregnancy, Rebecca Pearson and her colleagues at the University of Bristol in the UK investigated whether the ability to read faces varies during pregnancy.

They asked 76 pregnant women to assign one of six emotions to 60 computer-generated faces before the 14th week of pregnancy, and again after the 34th week. Faces expressing happiness and surprise tended to be correctly assigned at both stages of pregnancy, but for faces expressing fear, anger and disgust, the accuracy rates were higher in late pregnancy (Hormones and Behavior, DOI: 10.1016/j.yhbeh.2009.09.013).

This may increase the chance that the woman will spot potential threats to her and her fetus, and prime her to be hyper-vigilant once she becomes a mother. But it could have a downside. Pearson points out that people with clinical anxiety are also better at identifying negative emotions in faces. Pregnant women aren't clinically anxious, but "they might interpret negative or emotional things around them in a slightly more sensitive way", she says.

The finding builds on a recent study by Ben Jones of the University of Aberdeen in the UK who found that pregnant women - and women in stages of the menstrual cycle where progesterone levels spike - are better at identifying faces showing signs of sickness. "It's preventing them from becoming sick by interacting with people who are ill," he says.

The next step will be to examine whether pregnant women and new mothers are also more sensitive to emotional cues in babies' faces, Jones says.